The Swift compiler incorrectly sets the LLVM "ReadNone" attribute when
declaring swift_getObjCClassFromObject and swift_projectBox. This
means that the LLVM ARC optimizer will hoist ARC "release" operations
above these runtime calls. Since the implementation of the calls reads
the object, this causes a use-after-free crash.
This problem is easy to reproduce with
swift_getObjCClassFromObject. It was only exposed recently because the
SIL optimizer now shrinks object lifetimes, making it easier for LLVM
optimizations to kick in. It is difficult to expose the problem with
swift_projectBox, but the bug/fix is still obvious.
Fixes rdar://73820091: Use-after free application crash.
